In the conventional industrial robots, a motor is installed in an arm member, and a wrist member is connected through a hinge means to the outer end of the wrist member, in such a manner that the wrist member should be able to pivot by receiving power from the motor. When the wrist member performs pivoting movements by receiving the driving power from the motor, there are generated frictional abrasions due to the existence of the hinge means, with the result that it is difficult to maintain the concentricity. Further, when assembling the wrist, strong impacts are imposed on the joint portion, and therefore, efficient pivoting movements of the wrist can not be expected.
In order to overcome the above described disadvantage, there is proposed a wrist unit for industrial robot as shown in FIG. 1. As shown in this drawing, the wrist unit includes: an arm member 4 with an integrally formed guide portion 1 projected therefrom, with a hole 2 formed through a side of the upper portion, and with a portion formed thereon for receiving a motor 3; a wrist member 6 with a pair of projections 5 and 5' projected from the opposite sides thereof, and for performing pivoting movements about an axis A by receiving the driving power from the motor 3; and a side plate member 9 with a hole 7 formed for receiving the projection 5 of the wrist member 6, and secured on the other side of the upper portion of the arm member 4 by means of a bolt 8 to enable the wrist member 6 to perform pivoting movements without being dislodged. Further, bearings 27 and 27' are installed between the hole 2 of the guide portion 1 and the projection 5' of the wrist member 6, and between the hole 7 of the side plate member 9 and the projection 5 of the wrist member 6 respectively, so that the wrist member 6 should be able to perform pivoting movements in an efficient manner.
However, in the above described wrist unit, if the wrist member 6 is to perform pivoting movements in an efficient manner, the concentricity of the hole 2 of the guide section 1 and the hole 7 of the side plate member 9 has to be maintained above a certain precision. Therefore, when the wrist member 6 is assembled to the arm member 4 and the side plate member 9, first the side plate member 9 is secured to the arm member 4 by means of a bolt 8 and the like, and then, the holes 2 and 7 are formed simultaneously by machining the side plate member 9 and the guide portion 1. Then, in order to install the wrist member 6 having the projections 5 and 5', first the side plate member 9 has to be detached from the arm member 4 after the formation of the hole 7. Then the projection 5 of the wrist member 6 is inserted into the hole 2 of the guide portion 1, and then, the other projection 5' of the wrist member 6 is inserted into the hole 7 of the side plate member 9 after interposing a bearing 27' therebetween, thereby completing the assembling of the wrist member 6 into the arm member 4 and the side plate member 9.
That is, in the conventional wrist unit, the projections 5 and 5' are formed of one piece with the wrist member on the opposite sides of the wrist member 6, and the side plate member 9 has to be attached and detached before assembling the wrist member 6, with the result that the time required for the assembling is increased, thereby lowering the productivity. Further, the attachment and detachment of the side plate member 9 aggravates the concentricity of the holes 7 and 2, with the result that the pivoting function of the wrist member remains inaccurate as before.
In an attempt to overcome the above described disadvantages, Japanese Patent Laying-opening No. Sho-59-59392 proposes another wrist unit, and this wrist unit includes: a driving means supported by an arm; a first wrist arm with one end secured to a first revolution shaft (for transmitting the driving power); a second wrist arm with one end secured to a second revolution shaft (connected to the other end of the first wrist arm in a freely rotatable manner), and having a supporting portion in order to support a hand so as for the hand to be positioned at the same distance from the center of the second revolution shaft as the distance between the center of the first revolution shaft and the center of the second revolution shaft; a first revolution gear attached to the first revolution shaft in a rotatable manner; and a second revolution gear having a diameter equivalent to one half of that of the first revolution gear, with the first and second revolution gears being connected together.
However, in the above described wrist unit for industrial robot, the first revolution gear is attached to the base end of the first wrist arm in a rotatable manner, and the second revolution gear having a diameter of 1/2 of that of the first revolution shaft is also connected to the joining portion of the first wrist arm and the second wrist arm. Therefore it is possible to prevent the positional deviation when moving the hand, but the an excessive assembly time is required, and consequently, the productivity is reduced. Further, due to the deviation in the concentricity, the wrist member can not be moved in an accurate manner.